Electrical cables are ubiquitous in all electrical systems, for power supply or data transmission. As a reminder, an electrical cable usually consists of two metal conductors that follow the same path. A cable may be isolated or placed within a strand. These cables are subject to the same stresses as the systems that they connect and may be subject to failures or slow degradation. In many applications, it is therefore necessary to be able to test the state of these cables and their level of aging.
By way of illustration, cables that run along railroad tracks convey signaling instructions for rail traffic. They are very important and their use is critical for the smooth operation of the rail network. A failure may have repercussions on a major part of the rail network and result in train stoppages or delays that may accumulate and cause other delays. These cables are usually long and are subject to the effects of bad weather, spraying or mechanical effects such as, for example, vibrations from passing trains or landslides. They then age rapidly despite protection such as metal ducting or elastomer sheaths.
In nuclear power plants, electrical cables are subjected to ionizing radiation that threatens both the insulators and the metal and in the long run cause degradation in signal transmission qualities and reduced ability to withstand an accident. Although they are subjected to radiation, these cables must be able to withstand accident conditions such as massive irradiation over a very short time in order to be able to continue to ensure their function. Indeed, this function may be critical as, for example, supporting control signals for ordering actions following an accident, such as the release of control rods for stopping chain reactions.
In all these examples, knowing the state of aging of the cable enables anticipating the replacement of cables that may no longer function normally.
The ‘reflectometry method’ is the most widespread and effective method for solving the problem of knowing the state of aging. This method uses a principle similar to that of radar. An electrical, often high frequency or broadband signal, is injected in one or more places on the cable being tested. This signal is propagated in the cable or the network and returns a portion of its energy when it encounters an electrical discontinuity. An electrical discontinuity corresponds, for example, to a branch connection, to the end of the cable or to a fault. Analysis of the signals returned to the injection point can be used to deduce information therefrom on the presence and location of these discontinuities and therefore of possible faults.
As commonly used, the reflectometry method is particularly suitable for detecting and locating a fault located at one or more points along a cable. In the case of overall aging of the cable, it does not provide any relevant information.
In the remainder of the description, the term aging refers to the modification of the electrical parameters or physical characteristics along a cable or an assembly of cables, either homogeneously or otherwise. The term fault refers to a localized modification of these parameters, i.e. in a relatively small area compared with the length of the cable.